Palladium cyclic mechanism

Different approaches to the C5 cyclic mechanism have been put forward. Comparative studies of the isomerization of 2,2,3-trimethylpentane, 2,2,4-trimethylpen-tane, and 2,2,4,4-tetramethylpentane indicated161 that the latter did not display any appreciable isomerization on palladium below 360°C. Since this compound cannot undergo dehydrogenation without rearrangement to form an alkene, the cyclic mechanism on palladium is suggested to occur via the 1,2,5 triadsorbed species (20) bonded to a single surface atom. 

Kinetic studies on the palladium-catalyzed coupling of substrates such as 1-iodo-3,5-dichlorotrifluorobenzene with vinyl- or (4-methoxyphenyl)tributyltin carried out by the group of Espinet [225-228] led to a comprehensive proposal for the mechanisms of the Stille reaction that includes both open and cychc transmetallation steps (Scheme 1.29). The transmetallation in the cyclic mechanism involves an associative substitution (L for R ) through intermediate 54 and transition state TS5455 to give directly a cix-R /R complex 55, from which the coupled product will immediately lead to the formation of R -R by reductive elimination. A post-transmetallation intermediate cix-[PdR R L(ISnBu3)] in the cychc mechanism was spectroscopically detected [229]. 

As far as concerns the cyclic mechanism (Fig. 3.12), it consists in a two step reaction process in which the first one corresponds to the substitution of a ligand L by the stannane. Afterwards, the transmetalation reaction between the stannane and palladium occurs via a cyclic four-coordinated transition state resulting in a square planar complex where the two organic groups are in a cis arrangement. Then, in order to afford the coupled product and regenerate the catalyst, the reductive elimination reaction is required. 

To probe the reaction mechanism of the silane-mediated reaction, EtjSiD was substituted for PMHS in the cyclization of 1,6-enyne 34a.5 The mono-deuterated reductive cyclization product 34b was obtained as a single diastereomer. This result is consistent with entry of palladium into the catalytic cycle as the hydride derived from its reaction with acetic acid. Alkyne hydrometallation provides intermediate A-7, which upon cw-carbopalladation gives rise to cyclic intermediate B-6. Delivery of deuterium to the palladium center provides C-2, which upon reductive elimination provides the mono-deuterated product 34b, along with palladium(O) to close the catalytic cycle. The relative stereochemistry of 34b was not determined but was inferred on the basis of the aforementioned mechanism (Scheme 24). 

The study of hydrogen and deuterium electrosorption in palladium limited volume electrodes (LVE) was carried out by the same group in both acidic and basic solutions [124,130,134]. It was found that the hydrogen capacity, H (D)/Pd, measured electrochemically, depends significantly on sweep rate in cyclic voltammetric experiments and also on the thickness of the LVE. Two different mechanisms of hydrogen desorption, that is, the electrochemical oxidation and the nonelectrochemical recombination step, which take place in parallel within the Pd—LVE, have been postulated. 

Recently, Fu and coworkers have shown that secondary alkyl halides do not react under palladium catalysis since the oxidative addition is too slow. They have demonstrated that this lack of reactivity is mainly due to steric effects. Under iron catalysis, the coupling reaction is clearly less sensitive to such steric influences since cyclic and acyclic secondary alkyl bromides were used successfully. Such a difference could be explained by the mechanism proposed by Cahiez and coworkers (Figure 2). Contrary to Pd°, which reacts with alkyl halides according to a concerted oxidative addition mechanism, the iron-catalyzed reaction could involve a two-step monoelectronic transfer. 

A mixture of palladium chloride and triphenylphosphine effectively catalyzes carboxylation of linoleic and linolenic acids and their methyl esters with water at 110°-140°C and carbon monoxide at 4000 psig. The main products are 1,3-and 1,4-dicarboxy acids from dienes and tricarboxy acids from trienes. Other products include unsaturated monocar-boxy and dicarboxy acids, carbomethoxy esters, and substituted a,J3-unsaturated cyclic ketones. The mechanism postulated for dicarboxylation involves cyclic unsaturated acylr-PdCl-PhsP complexes. These intermediates control double bond isomerization and the position of the second carboxyl group. This mechanism is consistent with our finding of double bond isomerization in polyenes and not in monoenes. A 1,3-hydrogen shift process for double bond isomerization in polyenes is also consistent with the data. 

The stereochemistry of palladium-catalyzed hydrocyanation has been studied further using [Pd(DIOP)2] (133) as catalyst.607 It was shown that the addition of HCN to both cyclic and acyclic alkenes is cis. The mechanism is believed to be the same as for the nickel-catalyzed reaction (Scheme 58). 

The second Heck reaction involves a naphthyl iodide (Ar2 = 2-naphthyl) but the initial mechanism is much the same. However, the enol ether has two diastereotopic faces syn or anti to the aromatic substituent (Ar1) introduced in the first step. Palladium is very sensitive to steric effects and generally forms less hindered complexes where possible. Thus coordination of the palladium(II) intermediate occurs on the face of the enol ether anti to Ar1. This in turn controls all the subsequent steps, which must be syn, leading to the trans product. The requirement for syn p-hydride elimination also explains the regiochemical preference of the elimination. In this cyclic structure there is only one hydrogen (green) that is syn the one on the carbon bearing the naphthyl substituent is anti to the palladium and cannot be eliminated.. ... 

Higher temperatures and polar solvents are considered to switch the reaction mechanism of transmetalation from a four-centered transition state (Se2 (cyclic)) to a back-side attack of the palladium(II) complex (Se2 (open)) (Fig.1). 

The transmetallation step (iii) is certainly the most enigmatic part of the catalytic cycle. Generally, it is assumed to be rate limiting, and several mechanisms are proposed depending on the solvent. An open transition state with inversion of the stereochemistry would arise with polar solvents which are able to stabilize the transient partial charges , whereas a cyclic transition state with retention of the stereochemistry would arise in less polar solvents. It should be noted that the nature of the ligands on the palladium may influence dramatically the kinetics of the transmetallation step. A 1000-fold rate enhancement was observed when replacing triphenylphosphine by tri(2-furyl)phosphine . However, the dissociative or associative nature of the substitution on the palladium is stiU under discussion . ... 

A palladium-catalyzed 1,4-dialkoxylation of conjugated dienes was obtained when the 1,4-oxidation was performed in an alcohol as the solvent [105]. In this case it is necessary to run the reaction in the presence of a catalytic amount of a strong acid such as methanesulfonic acid or perchloric acid. Cyclic dienes gave a highly stereoselective 1,4-ds addition of the two alkoxy groups [Eq.(48)j. Also, the reaction with acyclic conjugated dienes proceeded in a, 4-syn addition. Thus, ( , )-hexa-2,4-diene gave E)- 2R, 5R ) dimethoxyhex-3-ene. The mechanism is depicted in Scheme 8-27. 

As described in Chapter 12, hydrogenation of cyclohexanone or related cyclic ketones over pre-reduced palladium hydroxide or palladium oxide in alcoholic solvents gave the ether as the primary product.35 The mechanism put forth for this reaction involved the intermediate formation of a ketal which was then hydrogenolyzed to the ether (Eqn. 18.12) 6 Evaporated platinum and palladium blacks, which have no basic impurities, promoted facile acetal formation. Further hydrogenation over palladium gave the ether as the almost exclusive product at a rate four times faster than that observed when reduced palladium hydroxide was used as the catalyst. Over the evaporated platinum catalyst only moderate amounts of the ether were formed. The primary product was the alcohol accompanied by some alkane.36 Ether formation was also observed on hydrogenation of cyclic ketones over Pt02 in ethanolic-HCl at room temperature and atmospheric pressure. Acetal formation occurred on... 

This reaction describes the entrance of a nucleophile into the allylic position of an olefin. In aqueous medium this reaction is of minor importance but in nonaqueous medium, particularly under the conditions of acetoxylation, it attracts broad interest. As already mentioned above and outlined later (see Section 3.3.14.6), higher and cyclic olefins give exclusively allylic esters. Two mechanisms have been proposed. One possibility is according to eq. (17) hydride abstraction through the palladium of an oxypalladation moiety by -elimination from the adjacent C-atom whieh had not been added to the nucleophile [9]. 

---